Functionally, however, the two reflex arcs can act synergistically, as the following example shows. Recall that the Golgi tendon organs monitor force levels across the tendon that they supply. If during maintained posture (such as standing at attention) knee extensors (such as the rectus femoris muscle) begin to fatigue, the force pulling on the patellar tendon declines. The decline in force reduces the activity of Golgi tendon organs in this tendon. Because the group Ib reflex normally inhibits the α motor neurons to the rectus femoris muscle, reduced activity of the Golgi tendon organs enhances the excitability of (i.e., disinhibits) the α motor neurons and thereby helps reverse the decrease in force caused by the fatigue. Simultaneously, bending of the knee stretches the knee extensors and activates the afferent fibers from the muscle spindles, which then excite the same α motor neurons. Thus coordinated action of afferent fibers from both the muscle spindle and Golgi tendon organ help oppose the decrease in contraction of the rectus femoris muscle due to fatigue and thereby work together to maintain the standing posture.
The flexion reflex starts with activation of one or more of a variety of sensory receptors, including nociceptors, whose signals can be carried to the spinal cord via a variety of afferent fibers, including group II and group III fibers, collectively called the flexion reflex afferent (FRA) fibers. In flexion reflexes, afferent volleys (1) cause excitatory inter-neurons to activate the α motor neurons that supply the flexor muscles in the ipsilateral limb and (2) cause inhibitory interneurons to inhibit the α motor neurons that supply the antagonistic extensor muscles (
Fig. 9.9).
This pattern of activity causes one or more joints in the stimulated limb to flex. In addition, commissural interneurons evoke the opposite pattern of activity in the contralateral side of the spinal cord (see
Fig. 9.9 ), which results in extension of the opposite limb, the crossed extension reflex. For lower limbs in humans (or for both forelimbs and hind limbs in quadrupeds), the crossed extension part of the reflex helps in maintaining balance by enabling the contralateral limb to be able to support the additional load that is transferred to it when the flexed limb is lifted.
Afterdamagetothedescendingmotorpathways,hyperactivestretchreflexesmayresultinspasticity,inwhichthereislargeresistancetopassiverotationofthelimbs.Inthiscondition,itmaybepossibletodemonstratewhatiscalledtheclasp-knife reflex. Whenspasticityispresent,attemptstorotatealimbaboutajointinitiallymeethighresistance.However,iftheappliedforceisincreased,therecomesapointatwhichtheresistancesuddenlydissipatesandthelimbrotateseasily.Thischangeinresistanceiscausedbyreflexinhibition.ThegroupIbreflexarcsuggeststhatrisingactivityinthispathwaycouldunderliethesuddenreleaseofresistance,andindeed,theclasp-knifereflexwasonceattributedtotheactivationofGolgitendonorganswhenthesereceptorswerethoughttohaveahighthresholdtomusclestretch.However,thetendonorganshavesincebeenshowntobeactivatedatverylowlevelsofforceandarenolongerthoughttocausetheclasp-knifereflex.Itisnowthoughtthatthisreflexiscausedbytheactivationofotherhigh-thresholdmusclereceptorsthatsupplythefasciaaroundthemuscle.Signalsfromthesereceptorscausetheactivationofinterneuronsthatleadtoinhibitionofthehomonymousmotorneurons.
Because flexion typically brings the affected limb in closer to the body and away from a painful stimulus, flexion reflexes are a type of withdrawal reflex. In